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Seals are a commodity to some users, but seals are the most critical element of the system for other users. If a $2 seal fails and causes $500,000 in repairs and/or warranty costs, it emphasizes how important that seal is to the system and its true cost.
Understanding the seal's role means understanding the components of the sealing system: the seal, the sealing surface (or shaft), the housing into which the seal is pressed, the lubricant and the outside environment (or what the seal is sealing out). Each component plays a critical role in the sealing system; therefore, each component needs to be considered when evaluating failures or designing a product.
This article will focus on lip seals for gear reducers, which are fairly common applications but can also be challenging. Typical input shaft speeds run about 1,750 rpm with output speeds ranging from 30 to 300 rpm. The output seal tends to be submerged in oil, while the input seal receives only a splash of oil. The input seal is typically more challenging than the output, but both deserve the necessary attention.
Consider a sample scenario in which the gear boxes are leaking. Using a system to deal with this problem, preliminary analysis points to an input seal leak. A great deal of critical information and data can be collected from the seal manufacturer before the unit is pulled apart. If the opportunity to collect that data is lost, it will make it much more difficult to determine the root cause. That said, if the unit is pulled apart before the seal manufacturer is contacted, follow the steps in the Rubber Manufacturer Association's (RMA) Sealing System Leakage Analysis Guide (OS-17). The first few pages of OS-17 offer a step-by-step guide for taking measurements and collecting critical data as the unit is pulled apart.
At this point, whether the data was collected when the unit was pulled apart or the seal manufacturer assisted in the failure analysis process, the data should point to the cause. For example, initial analysis might reveal that the seal was leaking from the inside diameter (or the shaft to seal interface). When the unit was pulled apart, the seal looked great (no excessive wear, no evidence of heat cracking, cuts in the lip, etc.). The next logical place to look is the shaft.
The running surface for the seal (shaft) is probably as critical, if not more critical, than the seal itself. The surface finish has several critical characteristics: Ra, Rz, Rpm and shaft lead (we will not discuss the specifics of these measurements and how they are calculated, but be aware of them and ensure they are all within their limits). It is essential to adhere to all these specifications to maintain integrity in the sealing system. Shaft Requirements for Rotary Lip Seals (OS-1-1) is another relevant publication. The key specifications are as follows: Ra: 8-17 µin, Rz: 65-115 µin, Rpm: 20-50 µin and shaft lead: < 0 ± 0.05 deg lead angle.
Staying with this example of leaking at the ID of the input seal, we will analyze the shaft in more detail. We pull the shaft, run some basic measurements and determine that the shaft is out of spec in Ra, Rz, Rpm and shaft lead. While this basically confirms why this particular seal leaked, we need to prevent this problem from getting worse. We take a random sampling of shafts from the shop floor (the shaft supplier can help if the shafts come from an outside source) and take measurements. In this hypothetical scenario, we pulled 37 shafts at random for measurement, collected the data, and did a capability study (see Figures 1 through 4). An overwhelming majority of these shafts are out of spec, which reveals a larger problem in the manufacturing process (or with the supplier).
Figure 1
Figure 2
Figure 3
Figure 4
In addition to microscopic defects on the shaft, Figure 5 shows more obvious signs of shaft damage that might be found during a shaft analysis.
Figure 5
Conclusion
Using the above tools to troubleshoot and/or prevent failures before they become problems is key. Work with suppliers and collect as much data as possible. This was just one example of one particular failure mode. In reality, it is not always easy to pinpoint something so obvious. The more data collected, the better chance of getting to the root cause of the problem.
In terms of troubleshooting, the OS-17 RMA guide can be used for more than just shaft failure analysis. It can be used to analyze bore failures, seal failures, oil failures and other common failure modes. In terms of prevention, understanding these failure modes upfront can also prevent problems. Understanding the interactions and specifications for all the components of the sealing system will help prevent failures as well (i.e. fluid compatibility, bore specifications and proper seal design). More information about this can be found in other RMA guides.
Pumps & Systems, November 2009
Nick Penze is president of Zerelli Technologies, Inc., Park West Finance Station, PO Box 20953, New York, NY 10025, 212-721-6769. Tags: Maintenance Minders , November 2009 Issue , Seals
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